Anti-clogging method for circulationheating of cooking liquid for digesters



8" 1957 E. A. ANNESTRAND 2,801,918

ANTI-CLOGGING METHOD FOR CIRCULATING-HEATING 0F COOKING LIQUID FOR DIGESTERS Filed Dec. 29, 1954 J 7 M 7 l7 3 13 l CQd INVENTOR. 6T1 finders a nneszrand BY IEMIW? 3M ATT R Y 2,801,918 Patented Aug. 6, 1957 ANTI-CLOGGING METHOD FOR CIRCULATION- HEATING OF COOKING LIQUID FOR DI- GESTERS Application December 29, 1954, Serial No. 47 8,423

Qlaims priority, application Sweden January 7, 1954 2 Claims. (Cl. 92-4 1 This invention relates to the preparation of pulp by passing a cooking liquid through a heat exchanger in encuit with a digester containing the raw material. It has particular reference to a novel method and apparatus by which clogging of the heat exchanger with pulpy material from the digester is prevented.

When preparing paper pulp from chip and simllar raw materials, the pulp is cooked by means of a cooking liquid (e. g. cooking acid) in special digesters. The heating of the cooking liquid is either direct or indirect. In direct heating, steam is introduced into the cooker. In indirect heating, the cooking liquid is led through a heat exchanger into which the steam is introduced. The digester forms a circulation circuit together with a circulation pump and the heat exchanger. The cooking liquid which is supplied to the heat exchanger is discharged from the middle part of the digester through at least one outlet thereof. Before arriving at the outlet of the digester, the cooking liquid passes a straining device, a circumferential sieve, arranged inside the mantle of the digester and is led to the heat exchanger by means of a circulation pump. The heated cooking liquid coming from the heat exchanger is usually led to the upper and lower ends of the digester. When the cooking is completed, the circulation through the system is interrupted and the pulp is drawn off through a valve in the bottom of the digester.

With such an arrangement, pulp fibers in the form of lumps are pressed through the circumferential sieve and remain therein. When the circulation is resumed during the following cooking operation, these lumps are carried by way of the pump to the heat exchanger and cause partial clogging of the latter. When a tube heat exchanger is used, the risk of such clogging generally is not so great. If, however, a plate heat exchanger is used, in which the throughflow channels are much smaller, the risk of the channels being clogged, and of a resulting burning-on of the heating surface, is so great that the utility of such heat exchangers for this purpose is greatly impaired, which is a serious disadvantage in view of the substantial advantages these heat exchangers have in other respects.

The present invention has for its principal object the provision of a method and apparatus for preventing the heat exchanger from becoming clogged with pulp fibers during circulation-heating (so-called indirect heating) of the cooked liquid for the digesters.

According to the invention, the cooking liquid discharged from the digester outlet connected to the heat exchanger is returned to the digester for a short period after the raw material of the pulp has been introduced into the digester but before the cooking liquid is heated in the exchanger, so thatduring this short period the circulation of the cooking liquid by-passes the heat exchanger, which is kept closed meanwhile. In this way, the lumps of pulp fibers which were previously pressed into the circumferential sieve in the digester, during the discharge of cooking liquid from the preceding cooking operation, are prevented from entering the heat exchanger and are instead led back directly to the opposite side of the straining device or sieve in the digester. These fiber lumps are thus caught by the raw material (chip pulp) in the digester. The circulation through the heat exchanger can therefore be subsequently resumed without any risk of the heat exchanger being clogged with agglomerated pulp fibers. The return of the pulp fibers to the digester, before the start of the cooking proper, is advantageously effected at the lowest point of the digester, since this makes it easier for the chip pulp to catch these fibers.

For a better understanding of the invention, reference may be had to the accompanying drawing, in which the single illustration is a schematic view of a preferred form of apparatus embodying the invention.

The apparatus as shown comprises a digester 1 of any common type which forms with pump 2 and heat exchanger 3 a circulation circuit for the cooking liquid. A straining device or sieve 4 is built into the digester 1. This sieve, as illustrated, is of annular shape so as to provide a free central space, the sieve forming with the surrounding wall of the digester an annular or circumferential space 4a through which the strained cooking liquid passes to outlets 5. From these outlets, the strained liquid is led to the heat exchanger 3 by means of the pump 2. From the heat exchanger 3, the cooking liquid is returned through a line 6 to the upper part of the digester. The heating medium, which may be steam, is led to and from the heat exchanger 3 by way of supply and discharge lines 7 and 8, respectively. The connecting line 9 leading from the pump 2 to the heat exchanger 3 has a branch line 10 returning to the lower part of the digester. In the return line 6 from the heat exchanger 3 to the digester 1 is a shut-off valve 11. The other lines may also be provided with shut-off valves 12, 13 and 14.

The apparatus is operated according to the invention as follows. Before a cooking operation is started, the digesting zone 1 is substantially filled with the chip or other raw material and cooking liquid, the valves 11 and 13 being kept shut. When the cooking is thereafter started, the valve 13 and the valves 12 are first opened. In this way, the cooking liquid is caused to circulate from the digester 1 through the straining device 4, the outlets 5, the pump 2 and the lines 9 and 10 back to the digester 1. The more or less agglomerated pulp fibers became lodged in the strainer 4 which during the preceding cooking, are consequently returned to the input side of the strainer 4 where they are caught by the fresh raw material (and the chips) pressed against the strainer. When all the fiber lumps have been returned in this way to the digester, the valve 11 is opened. The greater part of the cooking liquid is thus caused to circulate through the heat exchange zone 3 and is led through the line 6 back to the digester 1. The valves 14 for steam and condensate are thereupon opened so as to start the heating in the heat exchanger 3. When the cooking is completed, the valves 11 and 13 are shut and the relief gases are drawn oil in the usual manner from the digester. The prepared pulp is then drawn off through a line (not shown) arranged in the bottom of the digester. It is during this drawing-off of the pulp fibers after completion of the cooking that a part of the pulp fibers are pressed through the straining device 4, under influence of the over-pressure existing in the digester, and are pressed so as to form those lumps which during subsequent cooking cause the trouble mentioned above.

The installation shown on the drawing can be modified without departing from the invention. For example, the valve 11 may be placed in the line 9 between the return line 10 issuing therefrom and the heat exchanger 3,

lets 5 be positioned've rtically between those points of the digester 1 where the lines 6 and 10 are connected. Although the method and apparatus according to the invention are intended primarily to be used for preparing so-called sulphite pulp from wood chips, it will be understood that the invention is also applicable to other starting materials. The invention can be used to particular advantage in cases where the heating of the cooking liquid takes place in heat exchangers of the plate type.

I claim:

1. In the production of pulp by passing a cooking liquid through a heat exchanger of the plate type in circuit with a digesting zone containing the raw pulp material, whereby the liquid circulates from an outlet of the digesting zone through theheat exchanger and then back to the digesting zone, the improvement which comprises introducing fresh raw pulp material and cooking liquid into the digesting zone, and circulating all of the cooking liquid withdrawn from said outlet, under the influence of an over-pressure in the digesting zone, back to the digesting zone while by-passing the heat exchanger, prior to said circulating of the liquid through the heat exchanger, thereby preventing clogging of said heatzexchanger withpulp fibers.

2. The improvement according to claim 1, in which the cooking liquid circulated during said by-passing of the heat exchanger is returned to the bottom of the digesting zone.

References Cited in the file of this patent UNITED STATES PATENTS Morterud Nov. 14,1939 

1. IN THE PRODUCTION OF PULP BY PASSING A COOKING LIQUID THROUGH A HEAT EXCHANGER OF THE PLATE TYPE IN CIRCUIT WITH A DIGESTING ZONE CONTAINING THE RAW PULP MATERIAL, WHEREBY THE LIQUID CIRCULATES FROM AN OUTLET OF THE DIGESTING ZONE THROUGH THE HEAT EXCHANGER AND THEN BACK TO THE DIGESTING ZONE, THE IMPROVEMENT WHICH COMPRISES INTRODUCING FRESH RAW PULP MATERIAL ADD COOKING LIQUID INTO THE DIGESTING ZONE, AND CIRCULATING ALL OF THE COOKING LIQUID WITHDRAWN FROM SAID OUTLET, UNDER THE INFLUENCE OF AN OVER-PRESSURE IN THE DIGESTING ZONE, BACK TO THE DIGESTING ZONE WHILE BY-PASSING THE HEAT EXCHANGER, PRIOR 